Stateful Network Address Translation 64

The Stateful Network Address Translation 64 feature provides a translation mechanism that translates IPv6 packets into IPv4 packets and vice versa. The stateful NAT64 translator algorithmically translates the IPv4 addresses of IPv4 hosts to and from IPv6 addresses by using the configured stateful prefix. In a similar manner, the IPv6 addresses of IPv6 hosts are translated to and from IPv4 addresses through Network Address Translation (NAT). Stateful Network Address Translation 64 (NAT64) also translates protocols and IP addresses. The Stateful NAT64 translator enables native IPv6 or IPv4 communication and facilitates coexistence of IPv4 and IPv6 networks.

This document explains how Stateful NAT64 works and how to configure your network for Stateful NAT64 translation.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see
Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to
www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Prerequisites for Configuring Stateful Network Address Translation 64

For Domain Name System (DNS) traffic to work, you must have a separate working installation of DNS64.

Restrictions for
Configuring Stateful Network Address Translation 64

Applications
without a corresponding application-level gateway (ALG) may not work properly
with the Stateful NAT64 translator.

IP Multicast
is not supported.

Stateful NAT64
supports only cold redundancy. There are two redundancy mechanisms: cold
redundancy and hot redundancy. The redundancy mechanisms make the switchover of
NAT64 boxes transparent to IPv6 hosts. In cold redundancy, the mapping of
states is not synchronized among NAT64 boxes and the already established
connections are interrupted during a switchover of NAT64 boxes.

When traffic
flows from IPv6 to IPv4, the destination IP address that you have configured
must match a stateful prefix to prevent hairpinning loops. However, the source
IP address (source address of the IPv6 host) must not match the stateful
prefix. If the source IP address matches the stateful prefix, packets are
dropped.

Hairpinning
allows two endpoints inside Network Address Translation (NAT) to communicate
with each other, even when the endpoints use only each other's external IP
addresses and ports for communication.

Only TCP and UDP Layer 4
protocols are supported for header translation.

Routemaps are not
supported.

Application-level gateways
(ALGs) FTP and ICMP are not supported.

In the absence of a
pre-existing state in NAT 64, stateful translation only supports IPv6-initiated
sessions.

If a static mapping
host-binding entry exists for an IPv6 host, the IPv4 nodes can initiate
communication. In dynamic mapping, IPv4 nodes can initiate communication only
if a host-binding entry is created for the IPv6 host through a previously
established connection to the same or a different IPv4 host.

Dynamic
mapping rules that use Port-Address Translation (PAT), host-binding entries
cannot be created because IPv4-initiated communication not possible through
PAT.

Both NAT44
(static, dynamic and PAT) configuration and stateful NAT64 configuration are
not supported on the same interface.

Information About Stateful Network Address Translation 64

Stateful Network Address Translation 64

Stateful NAT64 supports Internet Control Message Protocol (ICMP), TCP, and UDP traffic. Packets that are generated in an IPv6 network and are destined for an IPv4 network are routed within the IPv6 network towards the Stateful NAT64 translator. Stateful NAT64 translates the packets and forwards them as IPv4 packets through the IPv4 network. The process is reversed for traffic that is generated by hosts connected to the IPv4 network and destined for an IPv6 receiver.

The Stateful NAT64 translation is not symmetric, because the IPv6 address space is larger than the IPv4 address space and a one-to-one address mapping is not possible. Before it can perform an IPv6 to an IPv4 translation, Stateful NAT64 requires a state that binds the IPv6 address and the TCP/UDP port to the IPv4 address. The binding state is either statically configured or dynamically created when the first packet that flows from the IPv6 network to the IPv4 network is translated. After the binding state is created, packets flowing in both directions are translated. In dynamic binding, Stateful NAT64 supports communication initiated by the IPv6-only node toward an IPv4-only node. Static binding supports communication initiated by an IPv4-only node to an IPv6-only node and vice versa. Stateful NAT64 with NAT overload or Port Address Translation (PAT) provides a 1:n mapping between IPv4 and IPv6 addresses.

When an IPv6 node initiates traffic through Stateful NAT64, and the incoming packet does not have an existing state and the following events happen:

The source IPv6 address (and the source port) is associated with an IPv4 configured pool address (and port, based on the configuration).

The destination IPv6 address is translated mechanically based on the BEHAVE translation draft using either the configured NAT64 stateful prefix or the Well Known Prefix (WKP).

The packet is translated from IPv6 to IPv4 and forwarded to the IPv4 network.

When an incoming packet is stateful (if a state exists for an incoming packet), NAT64 identifies the state and uses the state to translate the packet.

When Stateful NAT64 is configured on an interface, Virtual Fragmentation Reassembly (VFR) is configured automatically.

Prefixes Format for Stateful Network Address Translation 64

A set of bits at the start of an IPv6 address is called the format prefix. Prefix length is a decimal value that specifies how many of the leftmost contiguous bits of an address comprise the prefix.

When packets flow from the IPv6 to the IPv4 direction, the IPv4 host address is derived from the destination IP address of the IPv6 packet that uses the prefix length. When packets flow from the IPv4 to the IPv6 direction, the IPv4 host address is constructed using the stateful prefix.

According to the IETF address format BEHAVE draft, a u-bit (bit 70) defined in the IPv6 architecture should be set to zero. For more information on the u-bit usage, see RFC 2464. The reserved octet, also called u-octet, is reserved for compatibility with the host identifier format defined in the IPv6 addressing architecture. When constructing an IPv6 packet, the translator has to make sure that the u-bits are not tampered with and are set to the value suggested by RFC 2373. The suffix will be set to all zeros by the translator. IETF recommends that the 8 bits of the u-octet (bit range 64–71) be set to zero.

Well Known Prefix

The Well Known Prefix 64:FF9B::/96 is supported for Stateful NAT64. During a stateful translation, if no stateful prefix is configured (either on the interface or globally), the WKP prefix is used to translate the IPv4 host addresses.

Stateful IPv4-to-IPv6 Packet Flow

The packet flow of IPv4-initiated packets for Stateful NAT64 is as follows:

The destination address is routed to a NAT Virtual Interface (NVI).

A virtual interface is created when Stateful NAT64 is configured. For Stateful NAT64 translation to work, all packets must get routed to the NVI. When you configure an address pool, a route is automatically added to all IPv4 addresses in the pool. This route automatically points to the NVI.

The IPv4-initiated packet hits static or dynamic binding.

Dynamic address bindings are created by the Stateful NAT64 translator when you configure dynamic Stateful NAT64. A binding is dynamically created between an IPv6 and an IPv4 address pool. Dynamic binding is triggered by the IPv6-to-IPv4 traffic and the address is dynamically allocated. Based on your configuration, you can have static or dynamic binding.

The IPv4-initiated packet is protocol-translated and the destination IP address of the packet is set to IPv6 based on static or dynamic binding. The Stateful NAT64 translator translates the source IP address to IPv6 by using the Stateful NAT64 prefix (if a stateful prefix is configured) or the Well Known Prefix (WKP) (if a stateful prefix is not configured).

A session is created based on the translation information.

All subsequent IPv4-initiated packets are translated based on the previously created session.

Stateful IPv6-to-IPv4 Packet Flow

The stateful IPv6-initiated packet flow is as follows:

The first IPv6 packet is routed to the NAT Virtual Interface (NVI) based on the automatic routing setup that is configured for the stateful prefix. Stateful NAT64 performs a series of lookups to determine whether the IPv6 packet matches any of the configured mappings based on an access control list (ACL) lookup. Based on the mapping, an IPv4 address (and port) is associated with the IPv6 destination address. The IPv6 packet is translated and the IPv4 packet is formed by using the following methods:

Extracting the destination IPv4 address by stripping the prefix from the IPv6 address. The source address is replaced by the allocated IPv4 address (and port).

The rest of the fields are translated from IPv6-to-IPv4 to form a valid IPv4 packet.

Note

This protocol translation is the same for stateless NAT64.

A new NAT64 translation is created in the session database and in the bind database. The pool and port databases are updated depending on the configuration. The return traffic and the subsequent traffic of the IPv6 packet flow will use this session database entry for translation.

Stateful NAT64 utilizes configured access control lists (ACLs) and prefix lists to filter IPv6-initiated traffic flows that are allowed to create the NAT64 state. Filtering of IPv6 packets is done in the IPv6-to-IPv4 direction because dynamic allocation of mapping between an IPv6 host and an IPv4 address can be done only in this direction.

Stateful NAT64 supports endpoint-dependent filtering for the IPv4-to-IPv6 packet flow with PAT configuration. In a Stateful NAT64 PAT configuration, the packet flow must have originated from the IPv6 realm and created the state information in NAT64 state tables. Packets from the IPv4 side that do not have a previously created state are dropped. Endpoint-independent filtering is supported with static Network Address Translation (NAT) and non-PAT configurations.

Differences Between Stateful NAT64 and Stateless NAT64

The table below displays the differences between Stateful NAT64 and Stateless NAT64.

IPv6 systems must have IPv4-translatable addresses (based on RFC 6052).

ALGs supported

FTP64

None

Protocols supported

ICMP, TCP, UDP

All

Standards

Draft-ieft-behave-v6v4-xlate-stateful-12

Draft-ietf-behave-v6v4-xlate-05

State creation

Each traffic flow creates a state in the NAT64 translator. The maximum number of states depends on the number of supported translations.

Traffic flow does not create any state in the NAT64 translator. Algorithmic operation is performed on the packet headers.

High-Speed Logging for NAT64

Depending on your release, Stateful NAT64 supports high-speed logging (HSL). When HSL is configured, NAT64 provides a log of packets that flow through routing devices (similar to the Version 9 NetFlow-like records) to an external collector. Records are sent for each binding (binding is the address binding between the local address and the global address to which the local address is translated) and when sessions are created and destroyed. Session records contain the full 5-tuple of information (the source IP address, destination IP address, source port, destination port, and protocol). A tuple is an ordered list of elements. NAT64 also sends an HSL message when a NAT64 pool runs out of addresses (also called pool exhaustion). Because the pool exhaustion messages are rate limited, each packet that hits the pool exhaustion condition does not trigger an HSL message.

The table below describes the templates for HSL bind and session create or destroy. These fields (in the order they are displayed in the log) describe how the log collector must interpret the bytes in HSL records. The value for some of the fields varies based on whether the session is being created, destroyed, or modified.

Table 2
Templates for HSL Bind and Session Create or Destroy

Field

Format

ID

Value

Original IPv6 address

IPv6 address

27

Varies

Translated IPv4 address

IPv6 address

282

Varies

Translated IPv6 address

IPv4 address

225

Varies

Original IPv4 address

IPv4 address

12

Varies

Original IPv6 port

16-bit port

7

Varies

Translated IPv6 port

16-bit port

227

Varies

Translated IPv4 port

16-bit port

11

Varies

Original IPv4 port

16-bit port

228

Varies

Timestamp for an event

64 bits - milliseconds (This is a 64-bit field that holds the UNIX time, in milliseconds, when the event for the record occurred.)

323

Varies

VRF ID

32-bit ID

234

Zero

Protocol

8-bit value

4

Varies

Event

8-bit value

230

0–Invalid

1–Add event

2–Delete event

The table below describes the HSL pool exhaustion templates (in the order they are available in the template).

NAT translates any TCP/UDP traffic that does not carry source and destination IP addresses in the application data stream. Protocols that embed the IP address information within the payload (or in the application data stream) require the support of an ALG. ALGs handle application data stream (Layer 7) protocol-specific services, such as translating embedded IP addresses and port numbers in the packet payload and extracting new connection or session information from control channels.

FTP64 is automatically enabled when Stateful NAT64 is enabled. Use the
no nat64 service ftp command to disable the NAT64 FTP service.

Note

The FTP64 ALG is not supported in Stateless NAT64 translation.

Note

The FTP64 ALG does not support IPv4-compatible IPv6 addresses.

Based on IPv6-to-IPv4 translation FTP considerations draft-ietf-behave-ftp64-02 and RFC 2228, the FTP64 ALG must switch to transparent mode (a device in a transparent mode is invisible in the network; however, this device can act as a bridge and inspect or filter packets), when commands and responses flow between the FTP client and the FTP server. When a client issues the FTP AUTH command, the FTP64 ALG transparently forwards all data on the control channel in both (ingress and egress) directions, until the end of the control channel session. Similarly, during an AUTH negotiation, the ALG must be in transparent mode, whether the negotiation is successful or not.

Based on RFC 6384, the behavior of the FTP64 ALG during a client-server communication is different. During an IPv6-to-IPv4 translation, the FTP64 ALG must transparently copy data transmitted over the control channel so that the transport layer security (TLS) session works correctly. However, the client commands and server responses are hidden from the FTP64 ALG. To ensure a consistent behavior, as soon as the initial FTP AUTH command is issued by a client, the FTP64 ALG must stop translating commands and responses and start transparently copying TCP data that is sent by the server to the client and vice versa. The FTP64 ALG must ignore the AUTH command and not go into transparent mode if the server response is in the 4xx or 5xx ranges, which comprise FTP error/warning messages.

Prior to CSCtu37975, when an IPv6 FTP client issues an FTP AUTH command, irrespective of whether the IPv4 FTP server accepts or rejects that authorization negotiation, the FTP64 ALG moves the AUTH session to transparent mode (or bypass mode). When a session is in transparent mode, NAT cannot perform translation on the packets within the session. With CSCtu37975, during a client-server communication, the FTP64 ALG’s behavior is compliant with RFC 6384.

FTP64 NAT ALG Intrabox High Availability Support

Depending on your release, the FTP64 application-level gateway (ALG) adds high availability (HA) support for Stateful NAT64. The FTP64 NAT ALG Intrabox HA Support feature supports the stateful switchover between redundant Forward Processors (FPs) within a single chassis. The HA support provided by the FTP64 ALG is applicable to both intrabox HA and In-Service Software Upgrade (ISSU).

Use the
no nat64 service ftp command to disable the NAT64 ALG service.

The FTP64 ALG synchronizes data when it receives the following messages:

Fragment detection information after the first segmented packet is detected.

Fragment detection information after the end of the segmentation is detected.

Note

Stateful NAT64 supports only intrabox HA in some releases.

FTP64 ALG statistics and FTP64 debug logs are not synchronized to the standby device by the FTP64 ALG.

Stateful NAT64—Intrachassis Redundancy

Depending on your release, support for the Stateful NAT64—Intrachassis Redundancy feature is available. When a second Forward Processor (FP) is available inside a single chassis, the Stateful NAT64—Intrachassis Redundancy feature enables you to configure the second FP as a standby entity. When you plug in the second FP, redundancy starts automatically with no explicit configuration. There is a short delay before the standby FP becomes the “hot standby” (which means that all sessions have been synchronized). The standby FP maintains a backup of the Stateful NAT64 session information, and when the active (first) FP fails, there is very little disruption of NAT64 sessions.

NAT64 redundancy information is sent to the standby FP in the following instances:

When a session or a dynamic bind is created.

When a session or a dynamic bind is deleted.

During periodic updates. Based on the time elapsed, the active FP periodically updates the state information to the standby. Not all changes in the replicated objects are sent immediately to the standby at the time of change. The most critical updates are sent immediately, and other changes are communicated by periodic updates.

When a standby FP is inserted or when a standby FP recovers from a reload, the active FP performs a bulk synchronization to synchronize the standby FP with the active FP. NAT does an aggressive synchronization by which the active FP pushes all the state information forcefully to the standby FP.

In addition to NAT64 session information, application-specific information (application-level gateway [ALG] information) also has to be communicated to the standby FP. Each ALG has a per-session state that needs to be synchronized in the standby. The ALG triggers the sending of all ALG state information to the standby FP. NAT provides the mechanism for actually sending the ALG state and associates the state to a particular session.

HTTP sessions are not backed up on the standby FP. To replicate HTTP sessions on the standby FP during a switchover, you must configure the
nat64 switchover replicate http enable command.

Note

The Stateful NAT64—Intrachassis Redundancy feature does not support box-to-box (B2B) redundancy or asymmetric routing.

Configuring Static Stateful Network Address Translation 64

You can configure a static IPv6 address to an IPv4 address and vice versa. Optionally, you can configure static Stateful NAT64 with or without ports. Perform this task to configure static Stateful NAT64.

Defines the Stateful NAT64 prefix to be added to IPv4 hosts to translate the IPv4 address into an IPv6 address.

The Stateful NAT64 prefix can be configured at the global configuration level or at the interface level.

Step 16

nat64 v6v4 staticipv6-address ipv4-address

Example:

Device(config)# nat64 v6v4 static 2001:DB8:1::FFFE 209.165.201.1

Enables NAT64 IPv6-to-IPv4 static address mapping.

Step 17

end

Example:

Device(config)# end

Exits global configuration mode and enters privileged EXEC mode.

Configuring Dynamic Stateful Network Address Translation 64

A dynamic Stateful NAT64 configuration provides a one-to-one mapping of IPv6 addresses to IPv4 addresses in the address pool. You can use the dynamic Stateful NAT64 configuration when the number of active IPv6 hosts is less than the number of IPv4 addresses in the pool. Perform this task to configure dynamic Stateful NAT64.

SUMMARY STEPS

1.enable

2.configure terminal

3.ipv6 unicast-routing

4.interfacetype number

5.descriptionstring

6.ipv6 enable

7.ipv6{ipv6-address/prefix-length|prefix-name sub-bits/prefix-length}

8.nat64 enable

9.exit

10.interfacetype number

11.descriptionstring

12.ip addressip-address mask

13.nat64 enable

14.exit

15.ipv6 access-listaccess-list-name

16.permit ipv6ipv6-addressany

17.exit

18.nat64 prefix statefulipv6-prefix/length

19.nat64 v4 pool pool-name start-ip-address end-ip-address

20.nat64 v6v4 listaccess-list-namepoolpool-name

21.end

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ipv6 unicast-routing

Example:

Device(config)# ipv6 unicast-routing

Enables the forwarding of IPv6 unicast datagrams.

Step 4

interfacetype number

Example:

Device(config)# interface gigabitethernet 0/0/0

Configures an interface type and enters interface configuration mode.

Step 5

descriptionstring

Example:

Device(config-if)# description interface facing ipv6

Adds a description to an interface configuration.

Step 6

ipv6 enable

Example:

Device(config-if)# ipv6 enable

Enables IPv6 processing on an interface.

Step 7

ipv6{ipv6-address/prefix-length|prefix-name sub-bits/prefix-length}

Example:

Device(config-if)# ipv6 2001:DB8:1::1/96

Configures an IPv6 address based on an IPv6 general prefix and enables IPv6 processing on an interface.

Dynamically translates an IPv6 source address to an IPv6 source address and an IPv6 destination address to an IPv4 destination address for NAT64.

Step 21

end

Example:

Device(config)# end

Exits global configuration mode and enters privileged EXEC mode.

Configuring Dynamic Port Address Translation Stateful NAT64

A Port Address Translation (PAT) or overload configuration is used to multiplex (mapping IPv6 addresses to a single IPv4 pool address) multiple IPv6 hosts to a pool of available IPv4 addresses on a first-come first-served basis. The dynamic PAT configuration conserves the IPv4 address space while providing connectivity to the IPv4 Internet. Configure the
nat64 v6v4 list
command with the
overloadkeyword to configure PAT address translation. Perform this task to configure dynamic PAT Stateful NAT64.

SUMMARY STEPS

1.enable

2.configure terminal

3.ipv6 unicast-routing

4.interfacetype number

5.descriptionstring

6.ipv6 enable

7.ipv6{ipv6-address/prefix-length|prefix-name sub-bits/prefix-length}

8.nat64 enable

9.exit

10.interfacetype number

11.descriptionstring

12.ip addressip-address mask

13.nat64 enable

14.exit

15.ipv6 access-listaccess-list-name

16.permit ipv6ipv6-addressany

17.exit

18.nat64 prefix statefulipv6-prefix/length

19.nat64 v4 poolpool-name start-ip-address end-ip-address

20.nat64 v6v4 listaccess-list-namepoolpool-nameoverload

21.end

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ipv6 unicast-routing

Example:

Device(config)# ipv6 unicast-routing

Enables the forwarding of IPv6 unicast datagrams.

Step 4

interfacetype number

Example:

Device(config)# interface gigabitethernet 0/0/0

Configures an interface type and enters interface configuration mode.

Step 5

descriptionstring

Example:

Device(config-if)# description interface facing ipv6

Adds a description to an interface configuration.

Step 6

ipv6 enable

Example:

Device(config-if)# ipv6 enable

Enables IPv6 processing on an interface.

Step 7

ipv6{ipv6-address/prefix-length|prefix-name sub-bits/prefix-length}

Example:

Device(config-if)# ipv6 2001:DB8:1::1/96

Configures an IPv6 address based on an IPv6 general prefix and enables IPv6 processing on an interface.

Technical Assistance

Description

Link

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Feature Information for Stateful Network Address Translation 64

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to
www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

In Cisco IOS XE Release 3.5S, the FTP64 ALG adds HA support for Stateful NAT64. The FTP64 NAT ALG Intrabox HA Support feature supports the stateful switchover between redundant FPs within a single chassis. The HA support provided by the FTP64 ALG is applicable to both intrabox and interbox HA and In-Service Software Upgrade (ISSU).

Stateful NAT64 ALG—Stateful FTP64 ALG Support

Cisco IOS XE Release 3.4S

Cisco IOS XE Release 3.4S and later releases support FTP64 (or service FTP) ALGs. The FTP64 ALG helps Stateful NAT64 operate on Layer 7 data. An FTP ALG translates IP addresses and the TCP port information embedded in the payload of an FTP control session.

The following commands were introduced or modified:
nat64 service ftp.

Stateful NAT64—Intra-Chassis Redundancy

Cisco IOS XE Release 3.5S

Cisco IOS XE Release 3.10S

Cisco IOS XE Release 3.5S and later releases support the Stateful NAT64—Intra-Chassis Redundancy feature. When a second Forward Processor (FP) is available inside a single chassis, the Stateful NAT64 Intra-Chassis Redundancy feature enables you to configure the second FP as a standby entity. The standby FP maintains a backup of the stateful NAT64 session information and when the active (first) FP fails, there is no disruption of NAT64 sessions.

The following commands were introduced or modified:
nat64 switchover replicate http port.

Stateful Network Address Translation 64

Cisco IOS XE Release 3.4S

The Stateful Network Address Translation 64 feature provides a translation mechanism that translates IPv6 packets into IPv4 packets and vice versa. The Stateful NAT64 translator, algorithmically translates the IPv4 addresses of IPv4 hosts to and from IPv6 addresses by using the configured stateful prefix. In a similar manner, the IPv6 addresses of IPv6 hosts are translated to and from IPv4 addresses through NAT.

Glossary

ALG—application-layer gateway or application-level gateway.

FP—Forward Processor.

IPv4-convertedaddress—IPv6 addresses used to represent the IPv4 hosts. These have an explicit mapping relationship to the IPv4 addresses. This relationship is self-described by mapping the IPv4 address in the IPv6 address. Both stateless and stateful translators use IPv4-converted IPv6 addresses to represent the IPv4 hosts.

IPv6-convertedaddress—IPv6 addresses that are assigned to the IPv6 hosts for the stateless translator. These IPv6-converted addresses have an explicit mapping relationship to the IPv4 addresses. This relationship is self-described by mapping the IPv4 address in the IPv6 address. The stateless translator uses the corresponding IPv4 addresses to represent the IPv6 hosts. The stateful translator does not use IPv6-converted addresses, because the IPv6 hosts are represented by the IPv4 address pool in the translator via dynamic states.

NAT—Network Address Translation.

RP—Route Processor.

statefultranslation—In stateful translation a per-flow state is created when the first packet in a flow is received. A translation algorithm is said to be stateful if the transmission or reception of a packet creates or modifies a data structure in the relevant network element. Stateful translation allows the use of multiple translators interchangeably and also some level of scalability. Stateful translation is defined to enable the IPv6 clients and peers without mapped IPv4 addresses to connect to the IPv4-only servers and peers.

statelesstranslation—A translation algorithm that is not stateful is called stateless. A stateless translation requires configuring a static translation table, or may derive information algorithmically from the messages it is translating. Stateless translation requires less computational overhead than stateful translation. It also requires less memory to maintain the state, because the translation tables and the associated methods and processes exist in a stateful algorithm and do not exist in a stateless one. Stateless translation enables the IPv4-only clients and peers to initiate connections to the IPv6-only servers or peers that are equipped with IPv4-embedded IPv6 addresses. It also enables scalable coordination of IPv4-only stub networks or ISP IPv6-only networks. Because the source port in an IPv6-to-IPv4 translation may have to be changed to provide adequate flow identification, the source port in the IPv4-to-IPv6 direction need not be changed.